The invention relates to semiconductor manufacturing and more specifically to a method to condition a polishing pad on a chemical mechanical polishing (CMP) tool. A rigid pad conditioner is swept across an orbiting polishing pad to condition the polishing pad. In a preferred embodiment, the pad conditioner has a grid pattern of abrasives on its lower surface and the polishing pad is supported by a rigid platen.
A flat disk or xe2x80x9cwaferxe2x80x9d of single crystal silicon is the basic substrate material in the semiconductor industry for the manufacture of integrated circuits. Semiconductor wafers are typically created by growing an elongated cylinder or boule of single crystal silicon and then slicing individual wafers from the cylinder. The slicing causes both faces of the wafer to be extremely rough. The front face of the wafer on which integrated circuitry is to be constructed must be extremely flat in order to facilitate reliable semiconductor junctions with subsequent layers of material applied to the wafer. Also, the material layers (deposited thin film layers usually made of metals for conductors or oxides for insulators) applied to the wafer while building interconnects for the integrated circuitry must also be made a uniform thickness.
Planarization is the process of removing projections and other imperfections to create a flat planar surface, both locally and globally, and/or the removal of material to create a uniform thickness for a deposited thin film layer on a wafer. Semiconductor wafers are planarized or polished to achieve a smooth, flat finish before performing process steps that create the integrated circuitry or interconnects on the wafer. A considerable amount of effort in the manufacturing of modern complex, high density multilevel interconnects is devoted to the planarization of the individual layers of the interconnect structure. Nonplanar surfaces create poor optical resolution of subsequent photolithography processing steps. Poor optical resolution prohibits the printing of high-density lines. Another problem with nonplanar surface topography is the step coverage of subsequent metalization layers. If a step height is too large there is a serious danger that open circuits will be created. Planar interconnect surface layers are required in the fabrication of modern high-density integrated circuits. To this end, chemical-mechanical polishing (CMP) tools have been developed to provide controlled planarization of both structured and unstructured wafers.
CMP consists of a chemical process and a mechanical process acting together, for example, to reduce height variations across a dielectric region, clear metal deposits in damascene processes or remove excess oxide in shallow trench isolation fabrication. The chemical-mechanical process is achieved with a liquid medium containing chemicals and abrasive particles (commonly referred to as slurry) that react with the front surface of the wafer while it is mechanically stressed during the planarization process.
In a conventional CMP tool for planarizing a wafer, a wafer is secured in a carrier connected to a shaft. Pressure is exerted on the back surface of the wafer by the carrier in order to press the front surface of the wafer against the polishing pad in the presence of slurry. The wafer and/or polishing pad are then moved in relation to each other via motor(s) connected to the shaft and/or platen in order to remove material in a planar manner from the front surface of the wafer. Various combination of motions are known for moving the wafer and polishing pad in relation to each other, but typically the wafer is rotated and the polishing pad moved in either a linear, rotational or orbital manner.
For best planarization results, the polishing pad should be a uniform planar surface. However, various factors contribute to the non-planar shape of the polishing pad. During the manufacturing process of most polishing pads, each individual polishing pad is cut from a cylinder (cake) of polishing pad material. Imperfections in the surfaces of the polishing pads are created during the cutting process. In addition, windows, grooves, fluid holes and other processes are performed on the polishing pad prior to use that also create further imperfections in the surfaces of the polishing pads. These manufacturing imperfections are harmful to the planarization process.
Material is removed from the front surface of the wafer as the wafer is planarized. Some of the material becomes trapped in the polishing pad causing the polishing pad to become loaded and glazed with the material. This is a typical problem regardless of the type of motions undertaken by the wafer and polishing pad. However, conventional polishing pads moved linearly or rotationally do not have areas on the polishing pad that are continuously covered by the wafer. This allows deionized (DI) water, slurry or other fluids to be used to rinse away some of the material from all areas of the polishing pad, thereby reducing the problem. Orbital CMP tools, however, typically have areas of the polishing pad that are continuously covered by the wafer thereby reducing the ability to wash away material from the areas always covered by the wafer during the CMP process. Loading and glazing of the polishing pad for orbital tools are therefore particularly problematic. The polishing pad also loses material during the planarization process causing the polishing pad to lose its desired shape. It is typically desirable to have the surface of the polishing pad planar, but concave, convex and other shapes are also known in the art to be useful as contours for the polishing pad. Polishing pads are typically conditioned (reshaped and loaded material removed) between the time of unloading the old wafer and loading a new wafer. This allows the conditioning device to have complete access to the surface of the polishing pad.
Conventional conditioning devices have abrasives, most often diamonds, adhered to their bottom surface. However, Applicants have noticed that the random nature that the abrasives are fixed to the bottom surface of the conditioning device leads to nonuniform conditioning of the polishing pad. The bottom surfaces of conditioning devices used for orbital tools have been flexible, e.g. pneumatically supported or mounted to a flexible supporting surface. Prior art orbital CMP tools support the polishing pad with a flexible membrane. This allows the flexible conditioning device to conform to the flexible shape of the polishing pad and apply a uniform pressure against the polishing pad during the conditioning process. However, Applicants noticed that when a rigid surface was used to support the polishing pad on an orbital CMP tool, the conventional flexible conditioning device produced poor results. Specifically, Applicants noticed that the flexible conditioning device would xe2x80x9cchatterxe2x80x9d, i.e. skip along the surface of the polishing pad. The chatter produced gouges in the polishing pad and resulted in nonuniform conditioning of the polishing pad. In addition, Applicants noticed the flexible conditioning devices would bow and warp if not built very carefully resulting in non-uniform conditioning of the polishing pad.
Conventional orbital tools use one or more springs or weights to generate the pressing force needed to press the conditioning device against the surface of the polishing pad. Relative motion is generated between the polishing pad and the conditioning device to condition the polishing pad. The conditioning of the polishing pad removes the material and glaze on the polishing pad and reshapes the polishing pad to a desired contour. Applicants noticed that the springs did not provide adequate process control over the conditioning process. The springs tended to fatigue over time resulting in gradually lowering pressing forces over the lifetime of the springs. Also, the pad thickness would decrease over the lifetime of the polishing pad thereby unloading the springs and decreasing the pressing force.
What is needed is a method and apparatus for conditioning a polishing pad supported by a rigid platen on an orbital CMP tool that avoids the problems of the prior art. Specifically, a conditioning device is needed that properly shapes and uniformly removes the material loaded in the polishing pad quickly, without unnecessarily shortening the life of the polishing pad.
The present invention conditions a polishing pad, preferably supported by a substantially rigid platen, used during chemical mechanical polishing while avoiding the problems of the prior art. An object of the invention is to provide a conditioning device that does not warp or bow and uniformly conditions the polishing pad. The conditioning device includes a rigid elongated element that provides the strength necessary to resist torsional forces during the conditioning of the polishing pad that cause conventional conditioning devices to warp or bow.
Abrasive elements are supported by a convex or, preferably, a substantially planar bottom surface of the rigid element. The abrasive elements may have a diamond layer arranged into a grid pattern to provide a uniform abrasive surface. The conditioning device is preferably used to condition a polishing pad supported by a rigid platen. The conditioning device is pressed on and swept across the polishing pad by an actuator while the polishing pad is oscillated about an axis, rotated or orbited. The rigid conditioning device provides a uniform conditioning of the polishing pad. This uniform conditioning, while avoiding the bowing and warping of the prior art, provides a superior conditioning process.
In a typical process, a holder of wafers is loaded into a CMP tool. The wafers are sequentially taken from the holder and loaded into one or more carriers. The carriers press the front surface of the wafer against a polishing pad supported by a rigid platen. The wafer may be held stationary or rotated while the polishing pad is orbited to generate relative motion between the wafer and polishing pad to planarize the front surface of the wafer. The planarization process will load the polishing pad with waste material from the wafer reducing the effectiveness of the polishing pad. In addition, the polishing pad will lose its desired shape as the portions of the polishing pad that had greater contact with the wafer will experience a faster removal rate resulting in dishing of the polishing pad. Once the planarization process has been complete, the wafer is preferably cleaned, dried and replaced in its holder.
The polishing pad now needs to be conditioned to prepare it for the next wafer. This may be accomplished by pressing and sweeping a rigid conditioning device across the surface of the polishing pad supported by a rigid platen. While the actuator sweeps and presses the conditioning device against the polishing pad, a motion generator moves the platen and polishing pad in relation to the conditioning device. The preferred motion is an oscillation of the polishing pad in a clockwise and counter-clockwise direction. The range of the oscillation may be between about plus and minus 45 to 360 degrees and is preferably plus and minus 50 degrees. The oscillating motion of the polishing pad and the motion of the conditioning device uniformly remove the waste material loaded in the polishing pad and reshapes the polishing pad. The polishing pad is now ready for the next wafer and for the process to start again. It should be noted that the conditioning of the polishing pad may be performed after every wafer or after a predetermined number of wafers have been planarized.